The present invention relates to an extremely small one-chip switching power supply device formed of magnetic inductive elements, such as a transformer with a thin-film laminated construction and a reactor mounted on a semiconductor chip.
Switching power supply devices used widely as DC constant voltage power supply or stabilized power supply devices for electronic devices include a variety of circuitries, such as so-called forward-type, flyback-type and chopper-type devices. In the above systems, the input side and the output side are linked via magnetic inductive elements and devices, such as a transformer and an inductor, and a direct voltage on the output side is maintained constant at all times while turning on and off the input side current in the magnetic induction elements by switching elements such as transistors and controlling the duty ratio.
Since these switching power supply devices require rectification diodes, smoothing capacitors and controlling integrated circuit devices in addition to such magnetic induction elements and switching elements as described above, these circuit parts have all been mounted normally on a printed wiring substrate. However, as electronic devices continue to enlarge in scale and become more complex, wherein switching power supply devices with small capacities from several watts to ten watts are incorporated in electronic circuits for constituting such devices, switching power supplies that are as small and as inexpensive as possible have been in demand.
To meet such a demand, recent advanced high integration techniques can be utilized to incorporate all the semiconductor elements required in the switching power supply devices including switching elements and rectification diodes, in addition to the conventional control circuits into one single chip as small as 10 mm square or less in an integrated circuit device. In addition, the size of the magnetic induction elements and the smoothing capacitors can be decreased to nearly half of that in the conventional products by raising the switching frequencies for the circuit operation to several hundred kHz or higher, while elevating their effective reactance value.
Rationalization of the switching power supply devices has been previously advanced by integrating semiconductor elements and active elements on a single chip, and reducing the sizes of the transformers and smoothing capacitors by raising the switching frequencies as described above. However, these methods of solving the existing problems are approaching to their limit in terms of performance and reliability as explained below.
That is, even if the constituent parts are reduced in size and the number of parts is reduced, the way they are mounted on a printed wiring substrate is still the same. The mounting process is not eliminated even if the number of parts is reduced. Rather, the mounting work becomes more difficult as the parts become smaller, and hence the amount of labor actually required does not change much. In addition, since the active elements are connected to the passive elements via the wiring on the printed substrate, if the switching frequency exceeds 1 MHz, the device performance tends to vary because the circuit operation is affected by the wiring inductance, and the device tends to malfunction more easily because of incoming noise picked up by the wiring. Hence, the device reliability decreases. As a result, it is difficult to raise the switching frequency to about 1 MHz.
Another problem is that, when the switching frequency exceeds 1 MHz, the frequency characteristics of the magnetic inductive elements deteriorate to cause a saturation of the inductance value. That is, while it is possible to draw out an output in proportion to the square root of the frequency from the magnetic inductive elements with a certain size in a frequency region near 100 k Hz, and to obtain a reactance value proportional to the frequency, high-frequency loss increases in the magnetic circuits of the magnetic inductive elements in a high frequency region greater than 1 MHz, and the electrostatic capacity distributed internally increases. Therefore, since the frequency characteristic in the inductance value gradually deteriorates, and the reactance value which is a multiplicity of the inductance value and the angular frequency is saturated in a high frequency region greater than 10 MHz and increases very little, it becomes impossible to reduce the size of the magnetic inductive elements beyond a certain limit.
Still another problem is that, as the switching frequency increases, the high frequency loss in the switching element also increases. For example, the analysis results of losses in a switching power supply device of a flyback type operated at a switching frequency of 1 MHz indicate that the loss in the switching element accounted for 35% of the total loss, while the magnetic inductive element accounted for 20%, and other parts accounted for the remaining 45%. Meanwhile, the loss in the switching element is the largest, and this tends to form a bottleneck as the frequency is raised.
The present invention is intended to overcome such limits or the formation of the bottleneck, while providing a switching power supply device that is capable of further reducing the size and ensuring a high conversion efficiency.